Hybrid Resistance/Ultrasonic Welding System and Method

ABSTRACT

A hybrid resistance heating/ultrasonic welding method is used to join substrates. The resistance heating sufficiently softens or melts the substrates at an interface, and an ultrasonic wave is used solid state bond the substrates at the interface. The hybrid method can be used for both spot welding as well as continuous welding.

FIELD OF THE INVENTION

The present disclosure relates to a hybrid resistance/ultrasonic weldingsystem and method.

BACKGROUND OF THE INVENTION

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

As automotive technology has advanced, weight reduction requirementshave increased. In pursuit of these lower weight requirements,investigation into materials for use in automotive components that arelighter in weight and higher in strength has also increased. Materialssuch as aluminum, magnesium, and advanced high strength steels,therefore, are beginning to become more common in automotiveapplications. The use of these materials, however, has caused problemsin that these materials are generally difficult to join together bywelding.

For example, a material known as twinning induced plasticity (TWIP)steel shows dramatic improvement in both strength and ductility. TWIPsteel, however, contains carbon and manganese in a content that resultsin a carbon equivalent (CE) value that ranges from 3.33 to 4.7. The CEvalue is commonly used to evaluate the weldability of steel. When thisvalue exceeds 0.5, the material is considered difficult to weld. Becausethe CE value of TWIP steel is 6.7 to 9.4 times larger than more commonlyused steel sheets that are presently used in automotive applications,the weldability of TWIP is difficult. Accordingly, there is a need foran improved welding technology that makes it possible to join thelightweight and increased strength materials that are now considered foruse in automotive applications.

SUMMARY OF THE INVENTION

To satisfy the above need, the present teachings provide a spot weldingmethod that includes providing a pair of substrates, and applying anelectric current to the substrates to soften the substrates at aninterface between the substrates. After the substrates are softened, anultrasonic wave is applied to the substrates to solid-state bond thesubstrates at the interface.

The present teachings also provide a continuous welding method thatincludes feeding a pair of substrates through a first set of rollers.The first set of rollers are adapted to apply an electric currentthrough the substrates at an interface between the substrates. The pairof substrates are also fed through a second set of rollers. The secondset of rollers are adapted to apply an ultrasonic wave through thesubstrates at the interface. The substrates are subsequently joined atthe interface by applying the electric current and the ultrasonic waveto the substrates.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings disclosed herein are for illustration purposes only andthey are not intended to limit the scope of the present disclosure inany way.

FIGS. 1A and 1B are schematic cross-sectional representations of a spotwelding method according to the present teachings;

FIGS. 2A and 2B are schematic cross-sectional representations of a seamwelding method according to the present teachings;

FIGS. 3A and 3B are schematic cross-sectional representations of anotherspot welding method according to the present teachings; and

FIGS. 4A and 4B are a schematic cross-section representations of anelectrode/sonotrode used in accordance with the present teachings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses.

Referring to FIG. 1, a first embodiment of the present teachings willnow be described. In FIG. 1, a pair of substrates 10 and 12 are beingwelded together according to the method of the present teachings. Thesubstrates 10 and 12 are formed of advanced high strength steels (AHSS),and particularly twinning induced plasticity steel (TWIP). Although thepresent teachings are advantageous in welding AHSS such as TWIP steel,it should be understood that other materials that may be welding in thismanner include substrates formed from steel, stainless steel, aluminum(Al), magnesium (Mg), tungsten (W), titanium (Ti), cobalt (Co), silver(Ag), copper (Cu), brass, bronze, Fe-Austenite, nickel (Ni), platinum(Pt), platinum iridium (Pt—Ir), chromium (Cr), iridium (Ir),Fe-Martensite, molybdenum (Mo), niobium (Nb), tantalum (Ta), and otherdifficult-to-weld alloys such as Inconel, Monel, and nickel-based (Ni)superalloys. Substrates formed of dissimilar materials (i.e., onesubstrate formed of a first material and another substrate formed of asecond and different material) may also be joined together using thepresent teachings.

Additionally, metal substrates that include a coating such as zinc (Zn)or an oxide may also be used. In the present embodiment shown in FIG. 1,the substrates 10 and 12 are coated with Zn. The coating 11 and 13formed on the substrates 10 and 12, respectively, prevents rust andother materials from reducing the useful life of the substrates 10 and12. To join the substrates 10 an 12 together, a pair of electrodes 14and 16 pass an electric current 18 through the substrates 10 and 12.

The electrodes 14 and 16 press the substrates 10 and 12 with enoughforce to sufficiently ensure that no gap is between the substrates 10and 12 at an interface 19 where the substrates 10 and 12 are to bejoined together. By having no gap at the interface 19 between thesubstrates 10 and 12, there is sufficient contact between the substrates10 and 12 to ensure electrical conductivity between the substrates 10and 12. To press the substrates 10 and 12 together with the electrodes14 and 16, the electrodes 14 and 16 may be coupled to a device (notshown) such as a pneumatic or hydraulic device that is sufficient toensure face-to-face contact between the substrates 10 and 12 at theinterface 19. In this regard, the device should be capable of pressingelectrodes 14 and 16 against the substrates 10 and 12 with a sufficientforce to bring the substrates 10 and 12 into close contact. Preferably,the substrates 10 and 12 are pressed with the electrodes 14 and 16 witha force in the range between about 600-1200 lbs per square inch.Preferable devices include an air cylinder and a servo motor. Withrespect to a servo motor, this device is more preferable in that it isable to quickly apply and remove the force needed to press substrates 10and 12 together. In this manner, the force can be stationary or changedthroughout the welding process.

The magnitude of the electric current 18 is controlled to prevent, or atleast substantially minimize, melting of the substrates 10 and 12 fromoccurring. In this regard, the electric current 18 heats the substrates10 and 12, as well as the Zn coating 11 and 13 at the intended joiningarea or interface 19. It should be understood that controlling themagnitude of the electric current 18 is an important aspect of thepresent teachings because the Zn coating 11 and 13 has a melting pointless than a melting point of the substrates 10 and 12. By controllingthe magnitude of the electric current 18 passing through the substrates10 and 12, as well as the Zn coating 11 and 13, the substrates 10 and 12can be sufficiently heated to soften the substrates 10 and 12 withoutmelting them. Notwithstanding, the magnitude of the electric current 18is enough to reach the melting point of the Zn coating 11 and 13. Thecoating 11 and 13, therefore, is reduced to a molten form that isexpelled or “squeezed” out from between the substrates 10 and 12.

The coating 11 and 13 is expelled from the interface 19 between thesubstrate 10 and 12 at small gaps between the substrates 10 and 12located in areas outside of and adjacent where the force is applied bythe electrodes 14 and 16. Further, although not shown in the drawings,it should be understood that the coating 11 and 13 is sufficientlyheated at the electrode/substrate interface such that the coating 11 and13 is also expelled there. Moreover, it should be understood that theheating of the substrates 10 and 12 at the substrate/electrode interfacecauses thermal expansion at the substrate/electrode interface.Regardless, the force applied by the electrodes 14 and 16 is sufficientto enable face-to-face contact between the substrates 10 and 12 at theinterface 19, but at the area outside and adjacent the interface 19, thesubstrates 10 and 12 may bend upwards to allow a gap to form. This gapmay also be caused by thermal expansion of the substrates 10 and 12during application of the electric current 18 of the substrates 10 and12.

The preferred magnitude of the electric current 18 is preferably in therange of 2 kA to 30 kA, and more preferably in the range of 2 kA to 14kA. Although the ranges described above are preferred, one skilled inthe art will readily acknowledge and appreciate that the electriccurrent 18 should not be limited to the above ranges and can be set atany magnitude sufficient to soften any type of substrate known in theart. That is, although the present teachings are being describedrelative to joining substrates generally used in an automotiveapplication, the present teachings should not be limited thereto. Forexample, the present teachings may be adaptable to preparing electronicdevices where the substrates are formed of a material such as silicon(Si) or some other type of semiconductor material. In this regard, thecurrent 18 needed to sufficiently soften the substrates 10 and 12 willbe much less than the above-defined ranges.

By expelling the Zn coating 11 and 13 from the interface 19 between thesubstrates 10 and 12, intimate metal to metal contact between thesubstrates 10 and 12 is achieved. What's more, localized heating of thesubstrates 10 and 12 induces localized thermal expansion of thesubstrates 10 and 12, which in turn creates the gap and reduces thecontact area between the substrates 10 and 12 at the interface 19, whichis beneficial for concentrating the ultrasonic energy that issubsequently applied to the interface 19 in the desired bonding areabetween the substrates 10 and 12.

More specifically, as the electric current 18 is being passed throughthe substrates 10 and 12, as stated above, the electric current 18 heatsand softens the substrates 10 and 12. When the substrates 10 and 12 aresufficiently softened (plastically deformed), an ultrasonic wave 22 ispassed through the substrates 10 and 12 by a sonotrode 24 which causesthe substrates 10 and 12 to locally vibrate relative to each other at amicroscopic level at the interface 19 which generates friction betweenthe substrates 10 and 12 at the interface. The friction results in asolid state bonding between the substrates 10 and 12 to occur at theinterface 19. The ultrasonic wave 22 may be applied through thesubstrates 10 and 12 simultaneously with the electric current 18, orafter the electric current 18 has been applied. More particularly, itshould be understood that the electric current 18 is used tosufficiently soften the substrates 10 and 12. As such, so long as theultrasonic wave 22 is applied to the substrates 10 and 12 when thesubstrates are sufficiently softened by the electric current 18, animproved weld between the substrates 10 and 12 can be formed.

Although it is preferable to apply the ultrasonic wave 22 to thesubstrates 10 and 12 after the substrates 10 and 12 are sufficientlysoftened, certain applications require that the ultrasonic wave 22 beapplied to the substrates 10 and 12 before the electric current 18 isapplied. When joining substrates 10 and 12 that include an insulatinglayer (such as an oxide film) over a surface thereof, it is desirable toapply the ultrasonic wave 22 to the substrates 10 and 12 first. In thismanner, the vibration between the substrates 10 and 12 caused by theultrasonic wave 22 rubs the substrates 10 and 12 such that the oxidecoating is removed from the interface 19.

After the oxide coating has been removed from the interface 19, thesubstrates 10 and 12 have a more intimate face to face contact at theinterface 19. Subsequently, the electrodes 14 and 16 may apply thecurrent 18 to the substrates 10 and 12 to sufficiently soften them.During application of the electric current 18, the ultrasonic wave 22may continue to be applied, or cease to be applied until the substrates10 and 12 are sufficiently softened. Once the substrates 10 and 12 aresufficiently softened, the ultrasonic wave 22 may be re-applied to thesubstrates 10 and 12 to join them together.

The frequency of the ultrasonic wave 22 is preferably in the range of 20kHz to 35 kHz, and is generated by a sonotrode 24. The sonotrode 24 maybe any sonotrode commercially available. Alternatively, the electrodes14 and 16 may be configured to additionally act as sonotrode 24 bycoupling an ultrasonic generator (not shown), such as apiezoelectric-based device, to the electrodes 14 and 16. In anotherembodiment, a pair of sonotrodes may be used on opposite sides of thesubstrates 10 and 12, respectively.

By using a pair of sonotrodes, vibrations may be introduced tosubstrates 10 and 12 that are opposite in phase. In this regard,vibrations that are perpendicular and/or parallel to the substrates maybe introduced simultaneously to produce normal and shear frictionalforces at the interface 19. The normal and shear forces at the interface19 are advantageous in providing a more robust connection between thesubstrates 10 and 12 during the solid-state bonding process. Althoughperpendicular and parallel forces are described, it should be understoodthat forces with any arbitrary direction or orientation with respect tothe substrates 10 and 12 may be introduced. For example, rotational orangular vibrational forces may be introduced.

It should be understood that combining the use of resistance welding andultrasonic welding utilizes the benefits of each process. Morespecifically, the use of resistance welding softens the substrates 10and 12 to a point where plastic deformation of the substrates 10 and 12occurs. The plastic deformation of the substrates 10 and 12 allows thesubstrates 10 and 12 to come into intimate contact at the interface 19on a molecular level due to a rise in energy of the molecules of thesubstrates 10 and 12. That is, when the substrates 10 and 12 aresufficiently softened to plastically deform, the molecules of thesubstrates 10 and 12 begin to commingle at the interface 19 between thesubstrates. In addition to plastic deformation, the heat generated bythe electric current 18 also results in thermal expansion of thesubstrates 10 and 12 at the interface which enhances the commingling ofthe molecules.

It should also be understood that depending on the properties of thematerials to be joined, the ultrasonic wave 22 applied to the substrates10 and 12 may not have a sufficient energy to bring the substrates 10and 12 into a satisfactory bonding condition within the required timeframe necessary for physical manufacturing processing. By plasticallydeforming the substrates 10 and 12 prior to application of theultrasonic wave 22, the molecules of the substrates 10 and 12 aresufficiently excited such that the energy of the ultrasonic wave 22 issufficient to solid state bond the substrates 10 and 12. What's more,the vibrational forces applied by the ultrasonic wave 22 assist infurther commingling the molecules of the substrates 10 and 12 at theinterface 19. In this manner, the substrates 10 and 12 are joined with amore robust bond between them.

The above-described disclosure is advantageous in a spot welding method.The present teachings, however, should not be limited thereto. That is,the hybrid resistance heating/ultrasonic welding method is alsoapplicable to welding substrates 10 and 12 along a seam in a so-calledseam welding method. Referring to FIG. 2A, the electrodes 14 and 16 arein the form of rollers. By using rollers 14 and 16 configured to applythe electric current 18 between the substrates 10 and 12, the substrates10 and 12 can be pulled or pushed through the rollers 14 and 16 andstill be sufficiently heated to soften or melt the substrates 10 and 12.The rollers 14 and 16 can also be configured to act as the sonotrode 24that applies the ultrasonic wave 22 to the substrates 10 and 12 to solidstate bond the substrates 10 and 12 or control solidification of themolten nugget 26. In this manner, the electric current 18 and theultrasonic wave 22 can be applied simultaneously to the substrates 10and 12.

Alternatively, another set of rollers 15 and 17 configured to act as thesonotrode 24 can be used in conjunction with the rollers 14 and 16 thatact as electrodes to apply the electric current 18. Referring to FIG.2B, the rollers 15 and 17 may be disposed downstream (i.e., after) ofthe rollers 14 and 16 in a welding direction. Accordingly, the electriccurrent 18 may be applied by the rollers 14 and 16 and, subsequently,the ultrasonic wave 22 may be applied after the substrates 10 and 12 aresufficiently softened. It should be understood, however, that therollers 15 and 17 can be upstream (i.e., before) from the rollers 14 and16 such that the ultrasonic wave 22 can be applied to the substrates 10and 12 prior to the electric current 18 being applied. As stated above,such a method is advantageous when joining substrates 10 and 12 that maybe covered with an oxide coating. Although the pairs of rollers areshown disposed a distance from one another in the figures, it should beunderstood that the rollers may be disposed closer together. Further, itshould be understood that although the substrates 10 and 12 are shown ina seam welding application, the present teachings are also applicable tosubstrates in a lap seam welding configuration, a mash seam weldingconfiguration, and a butt seam welding configuration. Still further, thesubstrates 10 and 12 do not necessarily have to be the same thickness.That is, the substrates 10 and 12 can each have a different thicknesswithout departing from the spirit and scope of the present invention.

Now referring to FIG. 3A, a second embodiment of the present teachingswill be described. As shown in FIG. 3A, the same configuration as shownin FIG. 1A is depicted. Notwithstanding, in the second embodiment ahigher magnitude electric current 18 is passed through the substrates 10and 12. Because the electric current 18 is higher, the substrates 10 and12 are sufficiently heated to induce melting of the substrates 10 and12, which forms a molten nugget 26.

Once the nugget 26 is formed, the ultrasonic wave 22 is used to controlsolidification of the nugget 26. According to conventional weldingmethods, the solidified nugget 28 may have defects caused by a higheralloying element content or a higher CE value which may result inelement segregation, solidification cracks, or porosities. These defectsare undesirable in that when the molten nugget 26 solidifies, thestrength of the weld between the substrates 10 and 12 will not be asstrong as that required for automotive applications. Notwithstanding,according to the method of the present teachings, by applying theultrasonic wave 22 to the molten nugget 26 (see FIG. 3B), a stirringeffect is introduced inside the molten nugget 26 that can breaksolidification dendrites that may occur during solidification of thenugget 28. Because of the stirring effect caused by application of theultrasonic wave 22 during the solidification process, a uniform elementdistribution within the molten nugget 26 occurs, which results in asolidified nugget 28 with a uniform microstructure and a defect-freeweld.

It should be understood that the ultrasonic wave 22 is used to controlthe solidification process of the molten nugget 26. Due to the highalloy element content generally found in advanced high strength steels,the advanced high strength steel solidifies in a very large temperaturerange. During this interval, super cooling can occur, which may causesignificant dendritic growth and redistribution of the alloy elementswithin the solidified nugget 28. By applying an ultrasonic wave 22 tothe molten nugget 26, a stirring effect is occurring within the nugget26 which eliminates, or at least substantially minimizes, the supercooling and non-uniform element distribution which results in thedendrites being formed. By eliminating the non-uniform elementdistribution, and by that the constitutional super cooling, the presentteachings prevents severe detrimental residual stress in the weldbetween the substrates 10 and 12. Further, a shorter welding time andmore robust weld between these difficult-to-weld substrates 10 and 12 isenabled.

In the above-described hybrid resistance heating/ultrasonic weldingprocess, the electrodes 14 and 16 are used to compress the substrates 10and 12 at an interface 19. To further enhance the compression of thesubstrates 10 and 12, it should be understood that the electrodes 14 and16 can be provided with a plurality of teeth or a grooved surface thatassist in gripping the substrates 10 and 12. Referring to FIGS. 4A and4B, it can be seen that the electrode 14 includes a plurality of teeth30. Each of the teeth 30 include inclined surfaces 32. When theelectrode 14 is also adapted to act as a sonotrode 24, the profile ofthe teeth 30 can be used to influence a direction and orientation of thevibratory actuation force between the substrates 10 and 12 duringapplication of the ultrasonic wave 22. That is, as stated above, a pairof sonotrodes may be used to apply ultrasonic waves 22 with differingfrequencies to the substrates 10 and 12. In this manner, normal andshear forces that act perpendicular and parallel to the substrates 10and 12 can be formed during the ultrasonic welding process. The use ofthese normal and shear forces further enhances the solid state bondingbetween the substrates 10 and 12.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A method comprising: providing a pair of substrates with a contactinterface therebetween; applying an electric current to said substratesto soften said substrates at the interface; and applying an ultrasonicwave to said substrates to solid-state bond said substrates at theinterface.
 2. The method of claim 1, wherein said ultrasonic wave isapplied simultaneously with said electric current.
 3. The method ofclaim 1, wherein said ultrasonic wave is applied before said electriccurrent.
 4. The method of claim 1, wherein each substrate of said pairof substrates is selected from the group consisting of steel, stainlesssteel, twinning induced plasticity steel, aluminum, magnesium, tungsten,titanium, cobalt, and nickel-based alloys.
 5. The method of claim 1,wherein a first substrate of said pair of substrates is formed of afirst material and a second substrate of said pair of substrates isformed of a second different material.
 6. The method according to claim1, wherein at least one substrate of said pair of substrates includes acoating.
 7. The method according to claim 6, wherein during applicationof said electric current, said coating is expelled from said interfacebetween said substrates.
 8. The method according to claim 1, whereinapplication of said electric current melts said substrates at saidinterface to form a molten nugget.
 9. The method according to claim 8,wherein said ultrasonic wave is applied to said substrates to preventformation of defects in said molten nugget as said molten nugget coolsinto a solidified nugget.
 10. The method according to claim 1, whereinsaid electric current is applied to said substrates by at least oneelectrode.
 11. The method according to claim 10, wherein said electrodeis adapted to form a roller.
 12. The method according to claim 1,wherein said ultrasonic wave is applied by a sonotrode.
 13. The methodaccording to claim 12, wherein said sonotrode is adapted to form aroller.
 14. The method according to claim 1, wherein said ultrasonicwave is applied to said substrates at a plurality of frequencies. 15.The method according to claim 1, wherein said electric current isapplied to said substrates by an electrode, said electrode including aplurality of teeth.
 16. A method comprising: feeding a pair ofsubstrates through a first set of rollers, said first set of rollersadapted to apply an electric current through said substrates at aninterface between said substrates; feeding said pair of substratesthrough a second set of rollers, said second set of rollers adapted toapply an ultrasonic wave through said substrates at said interface; andjoining said pair of substrates at said interface by applying saidelectric current and said ultrasonic wave to said substrates.
 17. Themethod according to claim 16, wherein said second set of rollers isdisposed downstream in a weld direction from said first set of rollers.18. The method according to claim 16, wherein said second set of rollersis disposed upstream in a weld direction from said first set of rollers.19. The method according to claim 16, wherein application of saidelectric current either softens or melts said substrates at saidinterface.
 20. The method according to claim 16, wherein saidapplication of said ultrasonic wave solid state bonds said substrates atsaid interface.